Summary: A variety of immunostimulatory agents can have unexpected adverse consequences, such as the development ofautoimmune disease or the inmduction of tolerance. We examined the mechanisms responsible for the induction of these adverse consequences. Studies were designed to examine the immunostimulatory cascade initiated by protein antigens (including vaccines) and immunostimulatory CpG motifs present in DNA vaccines on the immune milieu, and whether they predisposed to the development of autoimmunity or tolerance. Our experiments show that agents capable of altering the balance between Th1 and Th2 cytokines profoundly impact the initiation, severity and persistence of both systemic and organ specific autoimmunity. The capacity of gp120-based vaccines and other protein-based immunogens to alter the cytokine milieu and thus contribute to the appearance of lupus-like symptoms was analyzed. We also showed (using an EAE model) that the administration of synthetic oligos expression CpG motifs predisposed to the development of organ specific autoimmunity (thourgh a Th1-mediated, IL-12 dependent process). Rrecent studies examined the ability of DNA vaccines to induce neonatal tolerance. We established the role of immature antigen presenting cells in this process, and showed that the co-administration of a plasmid encoding GM-CSF interferred with the developmetn of tolerance. We also documented the role of T cells in the establishment of this form of neonatal tolerance. In studies of the adjuvant-like properties of CpG motifs, we found that such motifs could promote the development of humoral and cgll-mediated immunity against the proteins and DNA vaccines with which they were co-delivered. Most recently, we confirmed that CpG oligonucleotides (being used as stand-alone immunomodulatory agents and vaccine adjuvants) could induce inflammatory arthritis when injected either locally or systemically. Of considerable interest, this adverse side effect was blocked by the co-administration of "suppressive" oligonucleotides. This provides insight into the role of CpG-containing bacterial DNA to the pathogenesis of autoimmune disease, and raises the possibility of such diseases being treated using suppressive ODN.